1. Field of Use
This invention relates to an improved multiple output switching power supply which provides a number of different regulated output voltages at the outputs of a number of secondary windings of a power transformer. A main output direct current voltage is derived from one of current flowing in the primary winding of the transformer. Auxiliary direct current output voltages are derived by rectifying the voltage at each of the other secondary windings and each secondary winding is controlled by its own regulator.
2. Prior Art
The most important requirements which must be satisfied by power supplies are efficiency, compactness, reliability and low radio frequency emission.
Great efficiency is usually attained by using transistors as switches in pulse width modulation circuits for controlling both the main output voltage and the auxiliary voltages. This is obtained with a trade off of more components, hence, cost and size increase.
In addition to the transistor and control circuit for each auxiliary voltage source, a storage filter is required between the secondary winding and the control transistor.
Compactness is usually attained by using high switching frequencies for the pulse width modulation process so as to reduce the size of transformer and filters. The switching frequency is limited by the performances of the transistors and by the trade off of less efficiency due to switching power losses in the transistors.
In order to attain compactness and cost reduction by elimination of some components, U.S. Pat. No. 4,122,514 discloses a D.C. power supply where auxiliary output voltages are controlled by SCR devices triggered by a control circuit responsive to the ripple of the auxiliary output voltage. A pulse width modulation of the pulsed power feeding the auxiliary output is obtained which overlaps the pulsed power feeding of the transformer, controlled by the main output voltage regulator. In this way, the storage filter intermediate to the auxiliary winding and the switching transistor of the auxiliary output is no longer required.
However, there are several disadvantages to this approach. The use of SCR limits the operating switching frequency of the power supply as compared to the switching frequency allowed by faster devices such as transistors and, more recently, power FETs (Field Effect Transistors). The SCRs and the primary winding switches are fully affected by the current spikes caused by the free-wheel diode associated with each output filter when such SCRs are switched on. The switching transistors controlling the current flowing in the primary winding of the transformer, must switch off the full equivalent current feeding both the main output section and all the auxiliary output sections.
Hence, large switching losses are caused in such transistors which must withstand thermal stress and/or must be provided with adequate and cumbersome heat dissipation devices. The large current ringing induced in the transformer at switch-off, causes a remarkable radio frequency emission which is required to be limited at an acceptable level by large sized snubbers.